1. Field of the Invention
The present invention relates to a method of stacking sheets sequentially discharged, a device for practicing the same, a sheet finisher using the device, and an image forming apparatus using the sheet finisher.
2. Description of the Background Art
It is a common practice with a copier, printer or similar image forming apparatus to staple or otherwise finish sheets carrying images thereon with a sheet finisher while discharging the finished sheets to a receipt tray or similar sheet stacking device. The sheet stacking device should be configured to stack a large number of stapled sheet stacks. However, the problem with the sheet stacking device is that staples sequentially piled up on the device raise the stacks and cause the top of the stacks to contact an outlet roller. The outlet roller contacting the top sheet stack is apt to catch it and disturb the sheets or, in the worst case, damage the sheets or cause them to drop. While the number of sheet stacks to be loaded on the sheet stacking device may be reduced for solving the above problem, this kind of scheme lowers productivity and cannot meet users' needs.
Further, the outlet roller constantly contacting the top sheet stack makes it impossible to pile further sheet stacks. As a result, the amount of sheets that can be stacked on the sheet stacking device is determined by the height of sheet stacks at the side where staples are positioned.
In light of the above, a receipt tray formed with concavity corresponding in position to a pile of staples has been proposed in the past. Although such a receipt tray may prevent a pile of staples from raising the sheet stacks, it is problematic when it comes to multistapling that is predominant today. More specifically, multistapling causes staples to be piled up at both sides of the trailing edges of sheet stacks. Moreover, to deal with various sheet sizes, the concavity must occupy major part of the trailing edge portion of the receipt tray for accommodating the piles of staples, resulting in irregular stacking or the drop of sheet stacks.
Japanese Patent Laid-Open Publication No. 2000-143082 discloses a method that shifts consecutive sheet stacks one by one for thereby preventing staples from raising the sheet stacks. This method has a problem that when curled sheets are stacked by being shifted to the side raised by staples, the raised side is further raised.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2000-86056 and 2000-327199.